The present invention relates to a dampening water composition for lithographic printing as well as an additive for dampening water for lithographic printing, which allows for a lithographic printing plate to provide printed matters having good quality by adding to the dampening water.
Lithographic printing technique makes the best use of the properties of water and an oil such that they are essentially incompatible with one another. The printing surface of a lithographic printing plate comprises areas which receive water and repel an oil ink and those which repel water and receive an oil ink, the former serving as non-image areas and the latter serving as image areas. The non-image areas become damp with dampening water used in lithographic printing which contains a desensitizing agent to thus enhance the difference in surface chemical properties between the image areas and the non-image areas and hence to increase both the ink repellency of the non-image areas and the ink receptivity of the image areas.
As such dampening water, there have generally been known conventionally aqueous solutions containing such inorganic substances as alkali metal salts or ammonium salt of bichromic acid, phosphoric acid or salts thereof such as ammonium salt, or such a colloidal substance as gum arabic or carboxymethyl cellulose (CMC).
However, it is difficult to uniformly dampen the non-image areas of lithographic printing plates with the dampening water containing such a desensitizing gum and for this reason, the resultant printed matters are sometimes contaminated and a substantial skill in controlling the feed rate of the dampening water is required.
To overcome such disadvantages, there has been proposed the Dahlgren dampening system in which an aqueous solution containing about 20 to 25% of isopropyl alcohol is used as dampening water. This method provides a variety of advantages concerning workability and accuracy of printed matters, such that the wettability of the non-image areas is improved, that the amount of the dampening water can be reduced, that it is easy to control the balance between feed rates of printing ink and dampening water, that the amount of water emulsified into the printing ink is lowered and that the transfer of printing ink to the blanket is improved.
However, isopropyl alcohol is apt to evaporate and, therefore, the use of a special device is required for keeping the concentration thereof constant. This is unfavorable from the economical point of view. Moreover, isopropyl alcohol gives out bad smell and is toxic and thus the use thereof is not favorable in view of the pollution of working atmosphere.
In addition, even if the dampening water containing isopropyl alcohol is applied to offset printing in which a dampening molleton roller is commonly used, isopropyl alcohol evaporates from a roller surface and the printing plate surface. Therefore, it cannot show its own effects.
Moreover, the pollution with industrial waste has become of a matter of great concern, the regulation with respect to discharge of chromium ions in waste water becomes more and more severer and there is a tendency of controlling the use of organic solvents such as isopropyl alcohol from the viewpoint of safety and hygiene. For this reason, it has been desired to develop desensitizing gums or dampening water free of such a compound.
Under such circumstances, Japanese Patent Publication for Opposition Purpose (hereunder referred to as "J. P. KOKOKU") Nos. 55-25075, 55-19757 and 58-5797 disclose compositions containing a variety of surfactants which can only slightly reduce the surface tension of water. In general, the dampening water should have a surface tension ranges from 35 to 50 dyn/cm. Therefore, if these compositions are used as dampening water, it is necessary to substantially increase the concentration of surfactants in such a desensitizing gum or dampening water. Furthermore, water is adhered to an ink film or an ink spreads over the surface of water because of vigorous movement of ink and/or water existing below an ink roll, a printing plate and a roll for supplying dampening water which rotate at a high speed, during the practical lithographic printing. However, combinations of surfactants disclosed in the foregoing methods explained above are insufficient for completely solve these problems. Besides, these dampening water containing such surfactants easily cause foaming during pumping and/or stirring thereof.
In addition, U.S. Pat. No. 3,877,372 discloses a solution containing a mixture of ethylene glycol monobutyl ether and at least one of hexylene glycol and ethylene glycol. U.S. Pat. No. 4,278,467 discloses a dampening water containing at least one member selected from the group consisting of n-hexoxyethylene glycol, n-hexoxydiethylene glycol, 2-ethyl-1,3-hexanediol, n-butoxyethylene glycol acetate, n-butoxydiethylene glycol acetate and 3-butoxy-2-propanol. Japanese Patent Un-examined Publication (hereunder referred to as "J.P. KOKAI") No. 57-199693 (U.S. Pat. No. 4,560,410) discloses dampening water containing 2-ethyl-1,3-hexanediol, Ester diol 204(viz., HOCH.sub.2 C(CH.sub.3).sub.2 CH.sub.2 OCOC(CH.sub.3).sub.2 CH.sub.2 OH), Hexyl Cellosolve or Hexyl Carbitol and at least one member selected from the group consisting of completely water-soluble propylene glycol, ethylene glycol, dipropylene glycol, diethylene glycol, hexylene glycol, triethylene glycol, tetraethylene glycol, tripropane glycol and 1,5-pentanediol. As these dampening water compositions do not contain isopropyl alcohol, they are preferable in view of safety and hygiene. However, the wettablility thereof with respect to non-image areas of a lithographic printing plate comprising an anodized aluminum substrate, during printing operation is not sufficient and it is sometimes observed that the non-image areas are contaminated, in particular, during high speed printing operation and that so-called ink spreading of half dot image portions, i.e., phenomenon wherein the shape of half dot images is abnormally deformed, is enlarged and is uneven, is caused. Moreover, 2-ethyl-1,3-hexanediol has not sufficient solubility in water and thus the use thereof is unfavorable to obtain a concentrated dampening water or an additive for dampening water having a high concentration.